1. Field of the Invention
The present invention relates to a sintered alloy material excellent in wear resistance and machinability, in particular, to a sintered alloy material which has distinctly excellent machinability and which is suitable for valve guides of internal combustion engines and manufacture thereof.
2. Related Art
In valve guides of internal combustion engines, special cast iron such as gray cast iron and boron cast iron can be used. However, in the case of cast iron, there are problems with working atmosphere, mass producibility and price, and thus substitution of sintered alloy for it is advancing. A general sintered alloy material is, however, poor in wear resistance, and improvement therefore is necessary. If an alloying component is blended to reinforce the material, the wear resistance of the material reaches a practical level, whereas the machinability thereof is deteriorated in many cases. The valve guide is attached to a cylinder head of the engine and subjected to a finish of the inner hole by reaming, before practical use. Thus, if the valve guide is poor in machinability, the time necessary for reaming may be prolonged and the wear of a reamer may also be advanced to decrease the efficiency of production.
The material for valve guides, which has developed previously by the applicant of the present application in an attempt at attaining both wear resistance and machinability (see Japanese Patent Application Publication (JP-B) No. 55-34858), is a sintered alloy having a composition consisting, by mass, of 1.5 to 4% carbon, 1 to 5% copper, 0.1 to 2% tin and 0.1 to 0.3% phosphorus and the balance iron. This material for valve guides is superior in wear resistance to boron cast iron and is also superior in machinability to conventional sintered materials, though it is harder to machine than cast iron materials. Therefore, it has been used widely by automobile manufacturer. However, owing to the recent change in the circumstances in this field, there is an increasing demand for improvements in qualities and for improvements in productivity as well, and as the material for valve guides, a material further excellent in machinability came to be required accordingly.
The present invention has been made with the above-described background in mind, and it is, therefore, an object of the present invention to provide a sintered alloy material for valve guides which has both wear resistance and machinability.
In order to achive the above-mentioned object, a sintered alloy material for valve guides, according to the present invention comprises: 1.5% to 4% by mass of carbon; 1% to 20% by mass of copper; 0.1% to 2% by mass of tin; 0.01% or more than 0.01% and less than 0.1% by mass of phosphorus; and an iron base, and having a metallographic structure comprising: a matrix phase comprising pearlite; and a free carbon phase being dispersed in the matrix phase.